1. Technical Field
This invention relates to light emitting diodes (LED) and more particularly, to devices having the inorganic thin film structure that an inorganic fluorescent substance between a pair of electrodes emits light.
2. Background Art
In recent years, light emitting devices have made a great advance. In particular, active research and development efforts are made on the following two subjects. The first relates to light emitting diodes (LED) and laser diodes (LD) relying as the basic principle on the injection and radiative recombination of electrons and holes across a semiconductor pn junction. The second relates to organic electroluminescent (EL) cells relying on the basic principle that an organic thin film serving as a light emitting layer is stacked along with electron transporting and hole transporting organic substances to accomplish injection and radiative recombination of electrons and holes similar to the semiconductor pn junction.
The LED and LD have long been studied while the research of GaN and ZnSe systems has marked a great advance in recent years. As shown in Nikkei Electronics, No. 674, page 79 (1996), for example, LEDs including a layered structure of such nitride semiconductor layers and emitting blue, green or similar light of short wavelength have already been developed. At present, reports relating to LD are also found though they are at an experimental phase. The reason why the LED and LD required a long time of development is that wide gap semiconductor materials such as GaN and ZnSe were easy to form n-type semiconductors, but difficult to form p-type semiconductors. With the progress of their crystal growth technology, the successful manufacture of p-type semiconductors was reported in recent years, which encouraged a rapid advance to realize LED and then LD.
However, LED and LD are difficult to apply as surface-emitting devices. Even if surface-emitting devices can be constructed, they become very expensive, suffering from a disadvantage from the price viewpoint. In the mass manufacture of blue-emitting devices, the crystal growth conditions, equipment, and single crystal substrates used are undesirably very costly as compared with red LEDs. It is currently believed that the market of blue-emitting devices expands 5 times if their cost is reduced to one half. It is urgently demanded to reduce the cost and improve the production yield of the relevant technology.
On the other hand, the organic EL devices can be formed on glass to a large area, on account of which research and development efforts have been made thereon to realize displays. In general, organic EL devices have a basic configuration constructed by forming a transparent electrode of ITO etc. on a glass substrate, stacking thereon a hole injecting and transporting layer of an organic amine compound and a light emitting layer of an organic substance exhibiting electron conductivity and intense light emission such as Alq3, and further forming an electrode of a low work function metal such as MgAg,
The device configurations which have been reported thus far have one or more organic compound layers interposed between a hole injecting electrode and an electron injecting electrode. The organic compound layers are typically of two- or three-layer structure.
In either structure, however, an unstable metal material having a low work function must be used as one of the electrodes (usually on the electron injecting side). None of the devices available thus far are satisfactory in device lifetime, luminous efficiency, ease of production, production cost, ease of handling and the like.
Since electroluminescent (EL) devices are light emitting devices having a thin fluorescent substance layer sandwiched between dielectric layers, they have several features including ease of handling of inorganic material, a wide angle of view, and a long lifetime. Further development of the device is expected.
However, since the luminous wavelength of the fluorescent substance used in EL is limited, the luminous wavelength band of EL devices is restricted, leaving the problem that a full color display and a specific color cannot be obtained. Inorganic fluorescent materials are difficult to provide a sufficient luminous efficiency, which becomes a serious obstacle to increasing the emission luminance and reducing the power consumption of devices. Further, the difficult adjustment of a fluorescent material makes difficult the representation of a subtle color and the application to large-area displays.